Optical discs are recording media which store video and/or audio information on their surfaces in the form of spirally arranged pits or relief information. The recorded information on such optical discs is reproduced through an optical pickup. Namely, the optical pick-up spots a light ray such as a laser beam or the like along the spiral recording track, while converting the reflected light into an electrical signal. A servo mechanism is usually employed to move the optical pickup with accurate tracing along the recording track.
Typical servo mechanism control methods include the so-called 3-spot method, push-pull method and DPD (Differential Phase Detection) method. In any one of these methods, a tracking error is detected from the reflecting position on an optical disc of a spot light ray from an optical pickup, the intensity of reflected light or the like. The tracking of the optical pickup is controlled by feeding back the tracking error signal to a control mechanism such as an objective lens or a mirror of the optical pickup device.
Shown in block diagram in FIG. 1 is an example of such a servo mechanism circuit, in which denoted at 1 is a rotating optical disc, at 2 an optical pickup capable of shooting a laser beam on the optical disc 1 while converting the reflected light from the optical disc 1 into electrical signals, at 3 an operational amplifier circuit for producing a tracking error signal according to the output of the optical pickup 2, and at 4 a drive circuit for driving a tracking coil provided in the optical pickup 2. The tracking coil drives an objective lens or a reflecting mirror to adjust the laser spotting position on the recording track of the optical disc 1.
The reference numeral 5 denotes an integral circuit which detects a DC component of the tracking error signal. After amplification by a power amplifier 6, the output of the integrating circuit 5 drives a thread feed motor 7 or a linear motor which moves the optical pickup 2 as a whole. Designated at 8 is a spindle motor which rotates a turntable, and at 9 is a feed screw.
When the disc 1 is flawless, the laser beam from the optical pickup 2 can trace the track correctly by the servo mechanism of the tracking servo control circuit. However, an instaneous drop of the intensity of the reflected light is caused, as if there were a tracking error, when there is a dust on the optical disc 1 or a defect or flaw such as an occluded air bubble in the recorded track. As a result, the optical pickup is caused to shift from the correct track to an adjacent track (i.e., a track jump), producing a large disturbance in the reproduced sound or image in some cases.
The instantaneous variation of the tracking error of this sort, which is caused by an abrupt drop in the reflected light intensity, can be prevented to some extent by decreasing the loop gain of the servo mechanism circuit. However, the reduction of the servo mechanism circuit loop gain makes it difficult to control the objective lens or reflecting mirror in conformity with impact or vibration externally applied to the reproducing apparatus.
For these reasons, it has been extremely difficult for the conventional servo mechanism circuit to maintain the optical pickup in correct tracking against both conditions of defects on optical discs and external mechanical disturbances.